1. Field of the Invention
The present invention relates to a digital RF converter, and more particularly, to a digital RF converter capable of improving a dynamic range of a transmitter and a signal to noise ratio, a digital RF modulator and a transmitter including the same.
2. Description of the Related Art
In wireless communication applications, the design thereof has been continuously developed to have simple and inexpensive wireless architectural structures which can increase integration in mobile terminals.
FIG. 1 shows a general analog circuit based transmitter. Referring to FIG. 1, the transmitter circuit includes digital-to-analog converters (D/A) 11 and 12, low pass filters (LPF) 13 and 14, mixers 15 and 16, a band pass filter (BPF) 17, and a linear power amplifier (PA) 18.
The digital-to-analog converters 11 and 12 convert in-phase signals I and quadrature signals Q of a baseband, respectively, which are discrete signals having a plurality of data bits, into analog signals while the low pass filters 13 and 14 filter spurious waves occurring at a position separated by a multiple of a sampling frequency of a baseband in consideration of digital signal characteristics.
The mixers 15 and 16 perform frequency up conversion on low-passed signals based on carrier signals (cos ωLOt, sin ωLOt) generated from a frequency synthesizer and the band pass filter 17 serves to filter the spurious waves that are not completely removed by the low pass filter or the spurious waves and cosine signals that are generated due to the non-linearity of the mixers.
The linear power amplifier 18 amplifies the filtered signals to generate the RF signals and transmits them through a duplexer or a switch to an antenna.
However, the transmitter shown in FIG. 1 has the following problem. First, the entire performance of the transmitter system may be deteriorated due to non-ideal operations of analog circuits such as non-linearity, carrier feedthrough, and so on, of the mixer and the low pass filter included in the transmitter circuit. Second, a bandwidth of a signal to be transmitted is limited due to analog baseband circuits. Third, when making a circuit including all the functions, an area occupied on the semiconductor substrate is increased.
In order to solve the problems, Shakeshaft (US 2005/01115330 A1) introduced the concept of a current steering D/A converter. According to the concept, Shakeshaft configures a digital-to-RF converter having both the function of a digital-to-analog converter and the function of a frequency up conversion mixer by connecting a plurality of cells in a Gilbert-cell mixer type in parallel as shown in FIGS. 2 and 3 and controlling each of them with digital signals and applies the digital-to-RF converter to a transmitter circuit system as shown in FIG. 4.
In Shakeshaft's invention, the dynamic range of the transmitting end is limited according to the number of cells connected in parallel, that is, a data bit size of the digital control signals connected to each cell. The maximum voltage of the output signal in the dynamic range is limited by the magnitude of the supply voltage to the circuit and the minimum voltage thereof is limited by the unit cell having the smallest size, that is, the size of the least-significant bit cell in FIG. 2, wherein the size is determined by the size of the transistor having the smallest sized gate width manufactured during the semiconductor process.
As described above, since the dynamic range is limited according to the number of cells, the maximum signal-to-noise ratio (SNR) that can be obtained by the transmitter circuit is limited by the restrictions in the semiconductor process and the supply voltage to the circuit.
Further, if each cell is designed to have the same unit-weight, the entire linearity of the transmission system is getting better, but the number of cells is increased such that the semiconductor circuit layout design becomes complicated and the electrical coupling in the circuit becomes large to increase the signal interference and the design area. When the data bit size of the digital control signal is, for example, 8 bits, the total number of required cells is 256.
Shakeshaft's invention uses two kinds of cells having different sizes to configure least-significant bit (LSB) sub-blocks by connecting large cells having a small operating current in parallel and most-significant bit (MSB) sub-blocks by connecting large cells having a large operating current in parallel as shown in FIG. 2, thereby reducing the total number of cells required in the transmitter circuit. If it is assumed that the current of the cell having the large operating current is 8 times larger than the current of the cell having the small operating current, the number of LSB cells configuring the LSB sub-blocks is 7 and the number of MSB cells configuring the MSB sub-blocks is 31, thereby making it possible to obtain a signal to noise ratio similar to that of the performance of a transmitter circuit configured of only 255 LSB cells.
However, even when the structure according to Shakeshaft's invention requires the performance of a transmitter circuit configured of only 1023 LSB cells, it requires 7 LSB cells and 127 MSB cells.